Semi-submersible vessels are large cumbersome vessels that need to be kept steady over a well site. They also need to be repositionable relative to certain defined coordinates. Many of the semi-submersibles currently in use are not provided with any propulsion system machinery, but are moved by tugs and held in position by anchor moorings. A need has existed for a system for dynamic positioning of these semi-submersibles without a major vessel conversion in a dock.
As oil and gas exploration is extending farther offshore into deeper water there is a need to convert many of the existing semi-submersibles from anchor moored vessels to dynamically positioned vessels.
Even in some shallow water areas, the use of anchor mooring systems may be prohibited, for instance, due to the presence of coral reefs or in locations where there already are multiple pipe lines and cables on the ocean floor and the use of anchors could damage the coral reefs or break existing pipe lines and cables.
A dynamic positioning system with externally mounted thrusters, each thruster having a self-contained power unit and a dedicated control system, has long been needed. A modular positioning system has been needed where the thrusters, power units, and controls are not integral with any of the semi-submersible's systems, nor are they integral with the hull of the semi-submersible, such that they allow easy attachment to a pontoon at sea and easy removal at sea when the system is no longer required. A need has existed for a detachable system so that expensive thruster systems can be leased rather than being owned by an operator, thereby at least theoretically lowering the cost of exploring for oil and gas, and lowering the cost of fuel for a consumer at the pump.
Additionally, a need has existed for a modular system that can easily be increased or reduced in overall size and capacity to suit semi-submersible vessels of different sizes.
A need has also existed for a fully packaged, self-contained thruster system that is fully integrated, factory tested and class approved prior to installation on the semi-submersible, allowing vessel upgrades of dynamic positioning capability within just a short period of time, and at a minimal cost.
A need has existed for a system which is easy to service at sea, allowing minimal down time without the need for a semi-submersible to return to a yard or dry dock, and allowing the semi-submersible to continue operating at its work location without interruption, thereby increasing the profitability of the operation.
The present embodiments are detailed below with reference to the listed Figures.